News from the Frontiers of Cosmology: A companion to the book The Edge of Physics

Tales of Russian ingenuity

by anil

Ice Fishing for neutrinos on a frozen Lake Baikal

WE HAVE ALL HEARD OF HOW NASA spend millions (or is it billions) on developing a pen that works in zero gravity, while the Russians used a pencil. A classic case of Russian ingenuity, it seemed, until it was exposed as an urban legend.

Russian ingenuity, however, is not a myth. I got to experience it first-hand while writing The Edge of Physics. One of the many trips I made for the book was to see the Lake Baikal Neutrino Telescope near Irkutsk, in Southern Siberia. The telescope is essentially long “strings” of photomultiplier tubes (PMTs) that are submerged more than a kilometer beneath the surface of the lake. PMTs can be thought of as the opposite of television tubes. A TV tube generates photons from electrical signals, while a PMT generates electrical signals from photons that hit its surface. The PMTs deep in the waters of Lake Baikal are looking for the blue Cherenkov light that is emitted when a neutrino hits a molecule of water.

So, where does Russian ingenuity come in? Well, for starters, they have figured out a way of deploying these detectors without the use of expensive ships and submersibles (as they do for the neutrino telescopes being built in the Mediterranean Sea). The Russians wait for Lake Baikal to freeze over, and then during the peak of the Siberian Winter, they establish an ice camp on top of the frozen lake. They bring their cranes and winches and the like, haul out their telescope from the depths of Lake Baikal, do the necessary maintenance and repairs, and get out of there before the ice melts.

Using this unusual and extremely hazardous mode of operation, they have managed to build the world’s first underwater neutrino telescope and run it for twenty years with only about $20 million. The other neutrino detectors, either underwater or embedded in the ice (such as at the South Pole), are costing hundreds of millions of dollars.

But the most telling illustration of Russian ingenuity – of course, necessitated by lack of resources sometimes, but worth appreciating regardless – was to do with the retrieval of a string that got cut one winter and sank to the bottom of the lake. Here’s a description of it from The Edge of Physics:

The Lake Baikal neutrino telescope is made of eleven strings of photomultiplier tubes—each with a large buoy at the top and a counterweight at the bottom—that float nearly 1.1 kilometers below the surface (the water here is a staggering 1.4 kilometers deep, enough for a building three times as tall as New York’s Empire State to sink without a trace). Smaller buoys attached to the strings float about 10 meters below the surface. All year round, a total of 228 PMTs watch for the Cherenkov light created by neutrinos, monitoring 40 megatons of water. Each winter, once the ice camp has been set up, the team has to locate the telescope, the upper part of which drifts slightly over the course of the year. A diver plunges into the ice-cold water to locate the small buoy fixed to the center of the telescope. Then the researchers cut holes in the ice above each string (whose positions they know relative to the center) and attach a winch to the small buoys to haul up the strings. The team has two months to carry out any routine maintenance, put the strings back in the water, and get out before the ice cracks. They have perfected their technique; only once in two decades of operation did they have a problem retrieving a string. In 1994, a rusty metal cable broke, severing the buoy from its string, causing the string to sink to the bottom.

Physicist Nikolai Budnev retrieved it. Diving that deep was out of the question, but Budnev knew that the string—though its counterweight was on the lake bed—would still be vertical because of the buoyancy of the PMTs. What he did next was ingenious. He fashioned a propeller and tied it to the end of a long rope, dropping the propeller into the water. The angle of the blades was such that as the propeller sank it started rotating, making huge circles. Budnev used this simple tool to sweep the waters below. Soon, the propeller snagged the errant string, and the team pulled it up.

I can confidently say that the Russians (and the Germans who worked alongside them) at Lake Baikal are amongst the toughest bunch of physicists I have encountered.